toolchain/wrapper: add option to print one argument per line

In case there are many arguments passed to the tools, the command line
can get very long, and difficult to parse visually.

For example, the Linux kernel passes a lot of arguments to gcc (at least
45, which gives 53 with our hard-coded args). Looking at such a command
line is daunting.

So, add the possibility to print each argument on its own line.

Also, enclose all args between single quotes, so the command line
can be safely copy-pasted without special chars (spaces, $) being
inrerpreted by the shell.

Add blurb about toolchain-wrapper to documentation at the same
time.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Thomas De Schampheleire <patrickdepinguin@gmail.com>
Acked-by: Luca Ceresoli <luca@lucaceresoli.net>
Tested-by: Luca Ceresoli <luca@lucaceresoli.net>
Acked-by: Samuel Martin <s.martin49@gmail.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>